The present invention broadly concerns devices and systems for use in the fixation of the spine and correction of spinal disorders. In one aspect, the invention concerns a spinal implant system utilizing elongated fixation elements, such as spinal rods, contoured for connection at various locations along the length of the spinal column. More specifically, the invention concerns a device for connecting the spinal rod to various vertebrae or the pelvis, while permitting variation in several degrees of freedom.
Several techniques and systems have been developed for use in stabilizing spinal curves and facilitating spinal fusion to correct spinal injuries and deformities. Frequently, an elongated spinal member, such as a bendable rod, is longitudinally disposed adjacent the vertebral column. The rod is fixed to various vertebrae along the length of the column by a number of fixation elements such as hooks or bone screws of various configurations. One example is the construct depicted in FIG. 1 which uses Danek Medical Inc.'s TSRH.RTM. spinal system. The fixation system 10 includes two elongated rods 11 disposed adjacent the spine on opposite sides of the spinous process. A variety of bone engaging fasteners 15 are engaged to the rods 11 by way of eyebolt assemblies 16. These eyebolt assemblies are known components of the TSRH.RTM. system and are particularly known for the "three-point shear clamp" effect achieved by the eyebolt assemblies to clamp the bone engaging fasteners 15 to the spinal rods 11.
It is often desirable to anchor the inferior end of the construct to either the sacrum or the pelvis to increase the stability of the construct. The rods of the system shown in FIG. 1 extends from the thoracic vertebrae to the sacrum and pelvis. In some prior art systems, this anchoring required bending the inferior end of the spinal rod which unfortunately has serious disadvantages. Bending the rod complicates the surgery, causes mechanical problems, and requires a more complicated spinal rod to be implanted. The Galveston fixation technique addressed this problem by providing a bend in the spinal rod. The construct shown in FIG. 1 uses an improvement of the Galveston technique in which a short rod segment 12 having a Galveston bend 14 is axially attached to the spinal rod 11.
As shown in FIG. 1, the system 10 is anchored to the iliac wings I by a pair of Galveston rod segments 12 each linked to the inferior end 11a of the respective spinal rod 11. The Galveston rod segments 12 each include an iliac extension 13 which is engaged within holes bores through the iliac wings. The Galveston rod segments 12 are engaged to the spinal rods 11 by way of a pair of offset plates 20 and eyebolt assemblies 21. The offset plate 20 axially links each Galveston rod segment 12 to the respective spinal rod 11. A transverse plate 17, preferably the Crosslink.RTM. plate provided by Danek Medical, is engaged between the two Galveston rod segments 12 by way of separate eyebolt assemblies 18. The addition of the Crosslink.RTM. transverse plate 17 adds greater strength and rigidity to the construct and prevents pullout of the iliac extensions 13 from the iliac wings I. This technique allows the substitution of a much shorter rod segment 12 already carrying the Galveston bend, thereby permitting ready engagement between the iliac wings and the spinal rods 11.
Although the use of the offset plates 20 has greatly simplified fixation in anchoring to the iliac bone from the prior Galveston technique, there still is room for improvement. In particular, rod bending is still required to create the iliac extensions 13. It is a known principal of mechanical engineering that bends in the rods yield stress concentrations and can also produce assymetric loading of the fixation construct. Moreover, the tools used to bend the rods can create notches in the rods which may compromise the overall strength of the construct.
It is therefore desirable to engage the bones of the sacrum or ilium without bending spinal rods. Custom fit capabilities such as angular and translational adjustability are also desirable. It is further desirable to provide such devices which can be readily connected to existing spinal fixation systems.